1. Field of the Invention
The invention relates to a dynamoelectric machine having a laminated stator core, and more particularly relates to the structure of short-circuiting rings and to the positioning of the short-circuiting rings for such machines.
2. Description of the Prior Art
In manufacturing stator cores for large dynamoelectric machines, laminations of the stator are formed in stacks and are pressed together between end flanges. The laminations are held in place by a plurality of key bars that are circumferentially spaced and extend axially beyond the laminations through the end flanges.
Key bars may carry large voltages induced within them as a result of stray leakage flux. Currents associated with key bar voltages pass through the key bars and circulate around the stator frame, core compression bands, section plates, flux shields and through other electrical paths within the core structure.
The induced voltages in adjacent key bars are not in phase, and currents passing between the key bars produce voltage drops between key bars. These voltage drops along with the induced voltages previously mentioned give rise to a resultant voltage impressed on the core laminations. This can cause core heating, especially if a point of contact exists in the core. When this heating becomes excessive, core damage or core failure may result.
In the prior art, short-circuiting rings have been employed outside the end flanges in close proximity to the flux shields to reduce these voltage differences. However, with the short-circuiting rings so positioned, there is interaction among the short-circuiting rings and the end flanges and the flux shields which are positioned outside the end flanges. Currents are induced in the short-circuiting rings as a result of this interaction, thereby reducing the effectiveness of the short-circuiting rings, at this location, in accomplishing their purpose of decreasing the voltage differences between key bars. Moreover, the impedance of the flange material to currents going through the key bars prevents the short-circuiting rings from achieving maximum effectiveness.
This problem of the prior art is overcome in the present invention by positioning the short-circuiting rings between the end flanges, rather than outside the end flanges. This avoids the aforementioned interaction and thereby improves significantly the ability of the short-circuiting rings to minimize voltage differences between key bars. By placing the short-circuiting rings between the end flanges the impedance of each flange does not interfere with the reducing of the voltage differences between key bars by its respective adjacent inboard short-circuiting ring. Moreover, a convenient arrangement is provided for accurately positioning the key bars in the desired location.
It is therefore an object of this invention to provide improved means to reduce the resultant voltage impressed on the core laminations and to minimize the risk of core failure.
It is another object of this invention to increase the effectiveness of short-circuiting rings by reducing interaction between the short-circuiting rings and other components of a dynamoelectric machine.
It is further object of this invention to substantially reduce induced voltage differences between the key bars.
It is yet another object of this invention to provide an improved arrangement for maximizing the efficiency of the short-circuiting rings.